PROJECT SUMMARY/ABSTRACT Disparities in breast cancer outcomes persist across race and socioeconomic status. Medicaid-insured women do not gain the same degree of survival benefit as those enrolled in other types of health insurance, but experience similarly poor prognosis as uninsured women. Younger, low-income patients face more barriers to access and adherence to high-quality cancer care as medical costs for cancer pose a more severe financial burden to younger compared to older patients. The barriers to high-quality cancer care for Medicaid-insured women have not been fully understood. Our preliminary data showed that Medicaid-insured women are highly mobile after breast cancer diagnosis. Despite the well documented effects of residential relocation on behaviors and health outcomes, there is lack of research that assesses the impacts of residential relocation after cancer diagnosis on patient's adherence to therapy and outcomes. Medicaid enrollees have inadequate access to specialist care. Much of adjuvant therapy for breast cancer is delivered in outpatient settings. As a result, coordination of multiple provider appointments and treatments is a significant challenge for residentially mobile patients. Residential relocation following cancer diagnosis may act as a key barrier to continuation and completion of cancer treatment for low-income patients and thus influences their prognosis. Drawing on our established collaborations with the Missouri Cancer Registry and the Missouri Medicaid Program, we will develop a contemporary population-based cohort of low-income women with breast cancer by linking the Medicaid claims to the Missouri Cancer Registry. Five other valuable data sources will be used to define characteristics of neighborhoods, providers and facilities. We will examine the impact of residential relocation on adherence to therapy in the first 12 months of diagnosis among low-income women with breast cancer (Aim 1). Residential relocation and its geographic patterns will be defined, which allows us to capture the dynamics of neighborhood exposures. We will also assess the associations between residential relocation and breast cancer outcomes in low-income women (Aim 2). It will control for residential mobility-induce spatial uncertainty and accurately estimate neighborhood impacts on cancer outcomes. Furthermore, we will examine the extent to which residential relocation explains racial disparities in adherence to therapy and prognostic outcomes in low-income women with breast cancer (Aim 3). This will be the first study to assess the role of residential relocation in adherence to breast cancer therapy and prognosis among low-income patients. It will deepen our understanding of the reasons for poor prognosis in medically underserved populations with breast cancer. The study will help guide breast cancer disparity interventions that could target residentially mobile, low-income patients and mitigate the adverse impacts of residential relocation on cancer treatment and outcomes.